Electron emitter comprising metal oxide-metal contact interface and method for making the same

ABSTRACT

The electron emitter herein disclosed comprises a tubular glass capillary substrate, on the interior of which is deposited an indium-doped tin oxide film. Gold ohmic contacts embrace the ends of the capillary, traversing the end edges and overlapping both the exterior of the capillary and the end margins of the film. A high-resistance region exists at the interface between at least one of the contacts and the oxide. It is formed by applying increased voltage across the contacts until there is a current increase independently of applied voltage, followed by a decrease in film conductivity.

United States Patent Soellner et al. 1451 May 16, 1972 s41 ELECTRONEMITTER COMPRISING 3,252,829 5/1966 Romstadt et a1 ..117/211 METAL()XIDE.METAL CONTACT 3,479,551 11/1969 McCann et al. ..313/334 gg zg FOROTHER PUBLICATIONS Borzjak et al., New Phenomena in Very Thin Films,"[72] Inventors: Arthur M. Soellner, Cape Girardeau, Mo.; Physica StatusSolid, Vol. 8, p. 55, 1965 James F, Sprouse, North Little Ro k; Tomchuket al., Emission of Hot Electrons from Thin Metal T i ith A, R j Lim Rk, b h f Films," Physics Institute, Academy of Sciences of the A k,UkrSSR, Kiev, Frzika Tverdogo Tela, Vol. 8, No. 1, pp. 276- 278,.lan.1966 [73] Assignee: Avco Corporation, Tulsa, Okla.

[22] Filed; Feb 13, 1969 Primary Examiner-David Schonberg AssistantExaminer-Toby H. Kusmer pp NW ,056 Att0rneyChar1es M. Hogan and EugeneC. Goodale 52 us. c1 ..313/339, 1 17/21 1, 313/334, [57] ABSTRACT313/346 The electron emitter herein disclosed comprises a tubular [51]Int. Cl. ..H01j l/20 gl pi y substrate. on the interior of wh h is po ie 531 Field 6: Search ..313/334, 339, 346; 1 17/21 1 an indium-doped tinOxide Gold Ohmic contacts embrace the ends of the capillary, traversingthe end edges and over- [56] References Cited lapping both the exteriorof the capillary and the end margins of the film. A high-resistanceregion exists at the interface UNITED STATES PATENTS between at leastone of the contacts and the oxide. It is formed b a lyin increased voltae across the contacts until there is 3,333,141 7/1967 Lemmens et a1..313/346 fll iicrease indepengenny of applied voltage, followed3,558,966 l/1971 H111 ..313/346 by a decrease in film conductivity3,264,074 8/1966 Jones .313/346 X 3,107,177 10/1963 Saunders et al ..117/21 1 1 Claim, 6 Drawing Figures INDIUM DOPED TIN OXIDE FILM gm mM UAH 7 O O m mAJ mm 4 MP T EOS & a U N s 6 w E m Dn 8 P R M U I v R SH 3 UEW M C n L I A Dlwm D T m m m m T m R W G Y B E 1 M t w h 4 s t e e .m w2 FILM CURRENT FURNACE CURRENT INDIUM DOPED TIN OXIDE FILM COLLECTOR d mw m BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention is an electronic emitter. The background literature on thefunction of electronic emission is extensive, and some of the morecommon methods are now mentioned.

Thermionic emission of electronics from hot cathodes, either filamentaryor indirectly heated, is a common part of the prior art and is welldescribed in the literature, for example:

Herbert J. Reich, Theory and Applications of Electron Tubes (New York:McGraw-Hill, 1944), pp. 19-42;

Frederick E. Terman, Electronic and Radio Engineering, 4th

' Ed., (New York: McGraw-I-lill, 1955), pp. 172-176;

M. I. T. Staff, Applied Electronics (New York: Wiley & Sons Co., 1943),pp. 74-98;

V. K. Zworykin, Television, 2nd Ed., (New York: Wiley & Sons Co., 1954),pp. 33-42.

Another common method for supplying electrons is photoemission, i.e. theemission of electrons from matter through excitation by light or otherradiant energy, as described in Zworykin, op. cit., pp. 42-49, or the M.l. T. text, pp. 101-109, for example.

The emission of ions in the disintegration of radioactive substances isutilized in glowand arc-discharge tubes as indicated in Reich, op. cit.,p. 11.

Another common method for supplying electrons is by secondary emission,certain substances giving off electrons when bombarded by impactingelectrons, as described in Terman op. cit., pp. 176-178, the M. I. T.text, pp. 109-112, and Zworykin op. cit., pp. 49-60. This phenomenon isutilized in electronic multipliers. 7

Field emission is produced by the action of intense electric fields at asurface, as described in the M. I. T. text, pp. 99-101.

2. Description of the Prior Art In recent years, advances in transistortechnology have caused increasing emphasis on semiconductor junctions assources of electrons. Among the advantages of such sources are longevityand emitter efficiency, a minimum of energy being dissipated as heat.

The prior art is characterized by low electron-emission efficiency,which is defined as the ratio of the emission current to the totalcurrent flowing in an electric. circuit. The electron emissionefficiency of the emitter in accordance with the invention commonlyattains values in excess of 50 percent and an efiiciency of 85 percent,with a collector current of 4 milliamperes was achieved in one instance.This compares to an efi'rciency of 2 percent to 5 percent for tunnelsources of the prior art.

A primary object of the invention is to provide a cold, solid statesource of electrons which is controllable and is characterized by a veryhigh electron emission efficiency.

Another object of the invention is to provide a source of electrons inwhich the thicknesses of certain thinly laid elements, described below,are not critical.

A further object of the invention is to provide a source of electronswhich can be formed in a variety of geometric configurations.

Still a further object of the invention is to provide means forgenerating a highly collimated beam of electrons.

In THE practice of the invention, very high emitter efficiencies havebeen achieved. The emission is instantaneous. It is known that currentfrom 10 to 200 microarnperes can be obtained. The electron stream can bemodulated at high frequencies. Power required is as low as 50milliwatts.

The emitter in accordance with the invention provides a very compactelectron gun and has numerous applications to mass spectrometry, gasanalyzers, cathode ray tubes, ultra high vacuum pressure gauges,multipliers, and plasma neutralizers, for example.

For a better understanding of the invention, together with other andfurther objects, advantages and capabilities thereof, reference is madeto the following description of the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing theprocess by which the oxide film is placed in the capillary incorporatedin a preferred embodiment of the invention;

FIG. 2 is a perspective view, partly in section, showing a preferredembodiment of electron emitters in accordance with the invention;

FIG. 3 is a perspective view of a preferred embodiment of the inventionin association with the circuit elements by which it is activated, asthe final step in the process of forming an electron emitter;

FIGS. 4 and 5 are sets of performance curves, with voltage as abscissaeand current as ordinates, on frameworks of Cartesian coordinates, andthese are used as an aid in explaining the invention; and

FIG. 6 is a perspective view of another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION AND THE PROCESSOF MAKING IT parameters were found to be as follows:

Length 6 centimeters Outer Diameter 6.35 millimeters Inner Diameter .75to 1.25 millimeters The hollow cylindrical laboratory-glass tubing 11 isprepared for incorporation in the finished product by cleaning in hotchromic acid, rinsing in distilled water, oven drying, and finallycooling to ambient temperature.

The invention is not limited to the use of a glass substrate. Thefunction of the substrate is to serve as an insulating medium, so thatany suitable insulating material, particularly a ceramic material, maybe used in lieu of glass.

The first step in the preparation of the invention, assuming readinessof the substrate, is the depositing of a thin film of indium-doped tinoxide on the substrate. This is accomplished by evaporation. Thesubstrate 11 is fitted to the output end of a delivery tube 12, theconnection being sealed by an asbestos seal 13, and the elements 1 l, 12and l3'are inserted into a furnace 14. Oxygen is employed as the carriergas and oxidizing agent. Oxygen is supplied by a suitable source 15,regulated by a flow meter 16, and passed through the delivery tube 12and the substrate 11 at the rate of 600 milliliters per minute. Thefurnace heats the substrate to a temperature of 700 C. Upon theattainment of these conditions the solution containing the desiredcomponents is injected into the oxygen path by a syringe 17. Thesolution, comprising approximately 0.1 to 1.0 milliliter of a 10 percenthydrochloric solution containing 0.2 M SnCl 5H 0 and 0.1 M lnCl wasinjected into the delivery tube by a hypodermic syringe.

After the injection of this solution the heating of the substrate 11 ismaintained for 2 minutes to assure complete depositing of theindium-doped tin oxide film. Then the capillary is removed from the ovenand allowed to cool to atmospheric temperature.

The doping with P-type semiconductor material, such as indium, isemployed in order to obtain higher resistance and to enhance theconditions for electron multiplication. In the practice of theinvention, pure tin oxide, tin oxide doped with antimony, and tin oxidedoped with indium have been found to be satisfactory.

After the depositing of the doped oxide film on the substrate, ohmiccontacts are provided by evaporating gold onto the ends of the filmedsubstrate. That is to say, the film is already along the interior of thesubstrate and gold ohmic contacts are superimposed on end margins of theoxide. Oxide film is indicated at 18. The gold contacts 19 and 20 aresubstantially alike. Each contact extends over the oxide film at the endmargin of the interior, as shown at 21, then traverses the end edge ofthe unit as shown at 22, and finally embraces the end margin of theouter periphery as shown at 23. The gold films l9 and 20 are depositedby vacuum evaporation. The areas on which gold is to be evaporated arebounded and defined by metal foil and plugs are inserted into theinterior of the unit in order to confine the depositing of gold to theend margins and to prevent the deposit thereof throughout the length ofthe interior. A suitable axial length of each gold contact is 7millimeters. The thickness of the gold contacts is not critical.

In lieu of gold any equivalent contact metal may be employed, preferablythe relatively inert metals in Group I B of the periodic table ofelements, such as copper or silver, i.e., a metal having good electricalconductivity. Aluminum and tin are also satisfactory.

Parenthetically, the initial resistances as measured between contacts 19and 20, prior to activation of the tin oxide film, i.e., prior tobuilding up the high-resistance region discussed below, indicated anohmic circuit. Values of 4 to 20 megohms were representative. Afteractivation, the resistance measurements went up to substantiallyinfinity and 900 megohms, respectively. The film thicknesses employedvaried from 0.1 to 10 microns. Parenthetically, the gold contacts can bedeposited by liquid expansion as well as by vacuum evaporation.

Reference is now made to FIG. 3 which shows suitable circuitarrangements for activation of the FIG. 2 embodiment, i.e., theproduction of the high-resistance region.

The capillary is placed within a glass container 24, as, for example,one 12 inches long and 4 inches in diameter. The chamber is evacuated toa pressure of 3 X 10 Torr. The vacuum is provided by vacuum pump 34. Ametallic collector 33 of disc-like configuration, approximately 0.75inches in diameter, is mounted approximately 0.25 inches in front of theend face of the unit.

Electrical lead-out connections 25, 26 and 27 are made with ohmiccontacts 19 and 20 and collector 33, respectively, and brought out tothe exterior of the tube 24. It will be understood of course that theseare provided prior to the evacuation of the tube. In circuit withcollector 33 and lead 27 are placed a current measuring device 28 and asource of direct current voltage 29, so poled as to impose a positivepotential on the collector. The meter 28 measures collector current. Incircuit with lead 26 and contact 20 are a current measuring device 30and a source 31 of direct current voltage, so poled as to render contact20 positive, i.e., the anode end. Total current is measured by a currentmeasuring device 32 in circuit between ground and lead 25 from contact19, i.e., the emitter or cathode end.

The collector 33 is preferably made of an alloy high in nickel content,containing iron, chromium and a trace of carbon.

The film deposition techniques utilized have the advantage that whilevacuum techniques are preferred, the depositions can be performed in airrather than in a vacuum for both the gold and the tin oxide films.Furthermore, it is not necessary to regulate the thickness of either thegold or tin oxide films since it was found that the emission does notcritically depend on these factors. The activating process consists inapplying a voltage across the metal-tin oxide films until a narrowhigh-resistance region fonns. Once this occurs, electron emission isobserved. Although generally performed in a vacuum, this activating orforming process is also accomplished successfully in air in a similarmanner. Following the formation of the highresistance region thecompleted capillary device can be stored in air for future use.

The high-resistance region is fonned in the following manner. A voltageis applied across the metal contacts 19 and 20 by variable voltagesource 31 and slowly increased. The film current through meter 30initially increases in a slightly nonlinear manner until a value ofanode voltage 20 is reached where film current in 30 continues toincrease independent of anode voltage. This is accompanied by anincrease in film temperature, generally up to 300 C. or more. An abruptdecrease of film current follows, often accompanied by arcing. Values offilm current, which, prior to the transition, are generally between 1and microamperes, measure between 0.1 and 2 microamperes after thetransition, indicating a decrease in film conductivity. A decrease inthe film temperature follows the transition. Values of anode voltage atwhich this transition occurs vary from 100 to 1,500 volts, dependent onthe initial resistance of the tin oxide film.

Following the transition, electron emission was always observed from anarrow high-resistance region, generally of a few microns width, whichformed in the tin oxide film adjacent to the positive gold film 20 andcompletely underlying the gold. The resistance of this region is usuallygreater than 50 megohms while the resistance of the remaining oxide filmis not materially changed. Concomitant with the electron emission is afaint violet luminescence accompanied by random scintillations in thehigh-resistance region of the tin oxide film.

The activating voltage at which this occurred was approximately 1,250volts. When a potential comparable to that on the anode end 20 of thecapillary is placed on the electron collector 33 by source 29, themicroammeter 28 in the electron collector indicates that an electroncurrent is flowing through it. This phenomenon occurs for either endused as the anode when the initial resistance is of the 4 megohm order.

Emission is verified by maintaining the capillary voltages at 20constant at values of 500, 750, 1,000, and 1,250 volts and varying thecollector voltage at 33 from 0 volts to values somewhat higher than therespective capillary voltage. See FIG. 4. Emission is also verified bymaintaining the collector voltage at 33 constant at values of 500, 750,1,000, and 1,250 volts and varying the capillary voltage at 20 from 0volts to values somewhat higher than the respective collector voltages.See FIG. 5.

With reference to the curves of FIGS. 4 and 5, it can be seen that thegreater the magnitude of voltage placed across the capillary, thegreater the amount of electron emission current which is measured at thecollector. For each value of capillary voltage used, the collectorelectron current saturates at a point where the collector voltage isapproximately equal to the capillary voltage. There is a range ofcollector voltage for each fixed capillary voltage where the collectorcurrent reverses direction.

The high-resistance region generally forms at the interface between theoxide film and the positive contact 20. As indicated, it can be formedadjacent the contact 19. The range of negative collector currentsindicated in FIG. 4 is caused by secondary emission from the collector,picked up at 20.

A collimated electron beam is obtained at the collector 33, the diameterof this collimated beam approximating that of the inside diameter of thecapillary. Since tin oxide films have a relatively high secondaryemission coefi'icient, these electrons are predominately secondaryelectrons produced along the inside diameter of the oxide film withinthe capillary.

It is of interest to note that an alternating current signal, up to highfrequencies, can be used to modulate the electronic emission from theend of the capillary.

The curves of FIGS. 4 and 5 relate to a specific embodiment of theinvention which was made and tested. In this embodiment the activatingvoltage, as stated above, had a value of approximately 1,250.

The activation process is accompanied by measurable increases inpressure but at the conclusion of the formation process the pressuredrops substantially to normal. Additionally, the temperature drops to apoint slightly in excess of ambient temperature. I

DETAILED DESCRIPTION OF A SECOND EMBODIMENT OF THE INVENTION clarity,having the following dimensions:

Length 3 inches Width 1 inch Thickness .l to .3 millimeters The filmsare deposited on the slide in substantially the same manner as has beendescribed with reference to the FIG. 2 embodiment. The depositingtemperature is approximately 600 C. The gold contacts may optionally beunderlaid by a relatively thin layer of gold of somewhat larger area,which relatively thin layer of gold is in immediate contact with themetal oxide. That is to say, extremely thin layers of gold may beevaporated onto the tin oxide and then two heavier layers 39 and 40applied for ohmic contacts.

The collector 38 is a metallic element similar to collector 33 and ismounted approximately 0.75 inch above the substrate 36. Electricalconnections are made in a manner similar to FIG. 3.

Application of a continuously increasing voltage between contacts 39 and40 of the FIG. 6 embodiment results first in a linear volt-amperecharacteristic, followed by a non-linear characteristic, a very rapidcurrent increase, arcing, and

finally an instantaneous drop, as described above. The indium doped tinoxide film in one embodiment had a film resistance of 250 megohms, andthe activating voltage was approximately 1,200 volts.

Emission is confirmed by maintaining the slide voltages constant at 500,750, 1,000 and 1,250 volts and varying the collector voltage from zerovolts to values somewhat higher than the slide voltages (the slidevoltage being that between the contacts 39 and 40). The emission wasconfirmed because collector current is observed.

The reference numeral 41 indicates the high-resistance region whichforms in the metal oxide film, under contact 40, for example. It will beunderstood that this high-resistance region is like that formed undercontact 20 in the FIG. 2 embodiment.

Emission is predominately in a direction parallel to the substrate 36,in the case of the FIG. 6 embodiment. This was confirmed by using amagnet to produce a deflecting and focusing action. In the case of theFIG. 2 embodiment emission is predominately in the axial direction.While the theory of action of both the FIGS. 2 and 6 embodiments neednot be disclosed herein, the evidence is strong that the emission is notthermal and the possibility of tunneling action is indicated.

Having fully disclosed our invention, we claim:

1. An electron emitter comprising:

a dielectric substrate,

a tin oxide film of a thickness between 0.1 and 10.0 microns disposed onsaid substrate, the tin oxide being doped with P-type material, and apair of spaced metallic contacts in contact with the oxide, there beinga high-resistance region between said contacts.

